Petrochemical industry is an important supporting industry in national economy, and supplies a large quantity of chemical raw materials for various departments including industry, agriculture, communication and national defense, which is thus one of the industrial sectors taking correlative and leading action in national economy. Lower olefins are one of the most important basic raw materials constituting modern petrochemical industry.
For instance, propylene is mainly used for the production of polypropylene, cumene, oxo alcohol, acrylonitrile, propylene oxide, acrylic acid, isopropanol and etc., wherein polypropylene accounts for more than half of the demand for propylene in the world. At present, 67% of propylene in the world is derived from by-products in the production of ethylene by steam cracking, 30% of which is derived from by-products in the production of gasoline and diesel oil by catalytic cracking unit (FCC) in refinery, and low amount of which (about 3%) is obtained from dehydrogenation of propane and metathesis reaction of ethylene-butylene. It is predicted that the demand of propylene in the future will be increased in a higher rate than the supply thereof.
Considering the relatively higher rate of increase in term of demand of propylene, and the situation of “demand exceeds supply” presented in conventional production modes, it is necessary to recur to other various new techniques of increasing yield of propylene for the purpose of supplementing the demand of propylene.
For many years, much experience has been achieved in the techniques of preparation of syngas from coal or natural gas, the preparation of methanol from syngas and separation of olefins for large-scale productions. Nevertheless, the process from methanol to olefins is still an unsolved and difficulty problem for the above industrial production chain from syngas to olefins. Solving the above key technique could provide a new resource approach for an industrial production of basic chemical materials, ethylene and propylene, from non-petroleum resources. In Particular, the demand for ethylene and propylene is continuingly increasing in recent years, while the petroleum resources are gradually exhausted. A development of a new coal-based chemical approach for the industrial production of lower olefins is of important strategic meaning and socioeconomic value in our country in greatly alleviating the situation of short petroleum supply, promoting a rapid progress of heavy chemical industry and constructive adjustment of resources.
The reference document CN1166478A disclosed a process for production of lower olefins such as ethylene and propylene from methanol or dimethyl ether. The process used an aluminophosphate molecular sieve as catalyst, and was an up-flowing dense phase circular fluidized process. Under the preferred reaction temperature of 500-570□, WHSV of 2-6 hr−1 and pressure of 0.01-0.05 MPa, methanol or dimethyl ether is cracked to produce lower olefins such as ethylene and propylene. The process, on the one hand, required a higher temperature, and on the other hand produced the target products with low selectivity. In addition, the fluidized bed technique has a technical defect of requiring relatively higher investment and operation costs.
The reference document CN1356299A disclosed a process and its system for production of lower olefins from methanol or dimethyl ether. The process used aluminophosphate molecular sieve (SAPO-34) as catalyst, and a gas-solid parallel down-flowing fluidized bed reactor for super-short contact, where the catalyst and the feedstock contacted with each other and the reactant stream was down-flowing. The obtained products and the catalyst flowed out of the reactor and then entered into the gas-solid rapid separator at the lower portion of the reactor for a rapid gas-solid separation. The separated catalyst entered into a regenerator to burn off the coke produced for regeneration. The catalyst was continuously regenerated in the system and the reaction was carried our continuously. The conversion of methanol was greater than 98% in the process. Nevertheless, the process equally presented a technical defect of requiring relatively higher investment and operation costs, and of lower selectivity to lower olefins.